Redox reactions & metabolism Lecture 6 Principles of - - PowerPoint PPT Presentation

Redox reactions & metabolism

Lecture 6 Principles of Microbiology for Engineers Dr Charles W. Knapp

Redox

• Oxidation – reduction reactions
• All chemical reactions where an atom has its
• xidative state changed
• i.e., CO2 versus CH2O versus CH4

(+4) + 2(-2) (0)+(+2)+(-2) (-4)(+4)

Reduction reactions

• Is the gain of electron, or decrease in oxidative

state

• Oxidant + electrons  product

(e.g.,) photosynthesis (in respects to C): 6CO2 + 6H2O + light  C6H12O6 + 6O2

Oxidation reactions

• Is the loss of electrons, or increase in oxidative

state

• Reductant  product + electons

(e.g.,) respiration (in respects to C): C6H12O6 + 6O2  6CO2 + 6H2O

In microbiology…

• Reduced chemicals represent “energy source”

– e.g., carbohydrates, S2-, NH3, Fe+2

• Oxidised chemicals represent “electron

acceptors”

– e.g., O2, SO4

2-, NO3

• , Fe+3

Half reactions

• In fact, each reaction represent a “half

reaction” (either oxidation or reduction).

• Reduction potential (E’0) can be determined.

Redox

• Reduction oxidation potential (redox) is a

measure of the ability of the environment to supply or receive electrons.

• The redox state of a solution is defined by the

number of free electrons present in the solution, in a fashion similar to pH

Redox

Redox

Gao, et al. (2003) California Agriculture 57: 55

Sabine Grunwald, Uni. Florida

Respiration

• Oxygen (O2) as the electron acceptor
• [CH2O] + O2  CO2 + H2O (+ ~5 ATP)
• Can generally use a great variety of carbon

substrates

• Different species are often specialised in terms
• f C-substrate use

Anaerobic respiration

• Electron transport is

analogous to aerobes, but use different redox potentials

• Nitrate – most common

form of anaerobic respiration (denitrification)

Anaerobic respiration

5[CH2O] + 4NO3

• + 4H+  5 CO2 + N2 + 7H2O
• Almost as energetically favourable as aerobic

respiration

• Significance to environmental engineering:

– N removal from systems – Production of greenhouse gases (N2O, NO)

Electron acceptors

• Metal oxides

– Abundant in soils and sediments of terrestrial origin – Iron oxides – Manganese oxides CH3COO- + 8 Fe3+ + 4H2O  2HCO3

• + 8Fe2+ + 9H+

Electron acceptors

• Metal oxides

– More limited range of C-substrates (e.g., no sugars)

• Acetate, lactate (simple organic acids)
• Simple alcohols
• Simple hydrocarbons
• H2

– Can use a great variety of metals & metalloids

• Cu, As, Mo, V, Cr, Se, Co

Electron acceptors

• Metal oxides

– Significance:

• Metal solubility changes with oxidation state
• Fe-oxides are solids  sorbs many other chemicals

Its reduction can lead to solubilisation and mobility of

• ther chemicals (e.g., phosphate, arsenic…)

Electron acceptors

• Sulphates

– Complete C-oxidisers – Incomplete C-oxidisers – Similar substrate conditions as metal reducers – Significance is primarily marine

80% of C-mineralisation in anaerobic sediment by sulphate-reducing bacteria

Electron acceptors

• Carbon dioxide

– e.g., methanogenesis – methanogens  all archaea 1) CO2 only: CO2 + 3H2  CH4 + H2O 2) Acetoclastic: CH3COO- + H2O  CH4 + HCO3

• (Acetogenic bacteria)

4H2 + 2HCO3

• + H+  CH3COO- + 4H2O

Electron acceptors

• Fermentation

– No electron-transport chain – (problem how to re-oxidise NADH?) – “Internal redox reaction”

NADH – nicotinamide adenine dinucleotide

Electron acceptors

• Fermentation

– Lactic acid fermentation (2-3 ATP/C) ATP is generated by substrate-level phosphorylation Strict redox balance: average oxidation state of product is same as substrates Only substrates with intermediate oxidation states can be fermented Must involve pyruvate as an intermediate Only under strict anaerobic conditions

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